1. Field of the Invention
The invention relates to circuitry for simultaneously biasing an electromagnetic transducer, particularly of the M-R type, and amplifying a signal generated by said transducer.
2. Description of the Prior Art
The use of electromagnetic transducers, particularly of the M-R type to reproduce pre-recorded data from a length of magnetic media is well known in the prior art. The scientific principle upon which an electromagnetic transducer of the M-R type operates is that an M-R element exhibits a change in resistance .DELTA.R as a function of an electromagnetic flux .phi. to which the element is exposed. The functional relationship between resistance and electromagnetic flux is utilized in adapting an M-R element to reproduce pre-recorded magnetic data.
Generally, the change in resistance .DELTA.R of an M-R device is an essentially non-linear function of the strength of the magnetic field H to which the device is exposed. In order to adapt the M-R device as an electromagnetic transducer and, particularly, a read transducer, it is desirable to center operation in the most linear region of the characteristic curve. This result is accomplished in the prior art by biasing the M-R device.
Prior art methods and apparatus for biasing M-R devices so as to enable said device to reproduce pre-recorded data may be broadly characterized into two classes. Each of the classes will be described hereinafter. The present invention which will be described hereinafter does not necessarily relate to apparatus and method characterized in the first class.
U.S. Pat. No. 2,500,953 issued Mar. 21, 1950 to M. L. Lisman and U.S. Pat. No. 1,596,558 issued Aug. 17, 1926 to B. N. Sokoloff are examples of the apparatus characterized in the first class. In this class of devices, the bias is supplied by a magnetic field generated from either an electromagnetic or a permanent magnet. Probably the most undesirable aspect with devices falling in this class is bulkiness due to the size of the magnets. The direction in which the technology moves is to fabricate an M-R transducer having relatively small size and less bulk.
This direction leads to the second class of prior art transducers. The present invention primarily relates to this class. With the advent of thin film technology the goal of fabricating an M-R transducer with reduced size and less bulk is realized. In essence, a first layer of thin film is deposited on a substrate with a second layer of thin film, sometimes referred to as the biasing film, deposited in proximity to the first film. An electrical current is applied to the second thin film. As a result of the current flow, an electromagnetic field is created which, in turn, biases the first film. Prior art examples of the second class of M-R transducers are described in U.S. Pat. No. 3,016,507 issued to Grant et al. on January 9, 1962, U.S. Pat. No. 3,366,939 issued to DeChanteloup on Jan. 30, 1968 and U.S. Pat. No. 3,678,478 issued to Copeland on July 18, 1972.
A further improvement in the second class of transducers, hereinafter called the thin film transducers, is achieved when a common circuit is used to generate the bias current and to bias a sensing circuit which processes signals outputted from the M-R transducer.
In one of the prior art schemes a resistive network is used to D.C. bias the M-R film and the sensing circuit. More particularly, two sections of an M-R transducer are interconnected to two balancing resistors to form a four-arm bridge circuit. The value of the balancing resistors are chosen to control bias current flowing through the M-R film as well as balancing the bridge. A more detailed description of the resistive approach to bias M-R transducers is described in U.S. Pat. No. 3,814,863 issued to R. L. O'Day et al on June 4, 1974.
In another prior art scheme, current sources are used for biasing. In this approach current sources are directly connected to the M-R film and supply the current for biasing said films. A more detailed description of the approach is given in U.S. Pat. No. 4,040,113 issued on Aug. 2, 1977 to F. W. Gorter.
Although the above described prior art biasing schemes work satisfactorily for the intended purpose, these schemes have drawbacks which the below described invention will solve.
One of the drawbacks of the prior art biasing scheme is that the balancing resistors must be necessarily larger than the resistance of the M-R film. This means that most of the excitation current is dissipated in the balancing resistors.
Another drawback which plagues the prior art biasing scheme is that whenever current source is used as the biasing means, an unusual amount of noise is generated by the current source. The additional noise adversely affects the overall system performance.
A common problem which is associated with either resistive biasing or current source biasing is that an offset voltage is generated across the input of the circuit which processes the signal outputted from the M-R transducer. This offset voltage tends to saturate a preamplifier which is usually used in the processing circuit.
To solve the offset voltage problem, the prior art adapts A.C. coupling instead of D.C. coupling to connect the M-R transducer to the processing circuitry. A.C. coupling requires additional components such as coupling capacitors, etc. The effect of A.C. coupling reduces the simplicity of the system and increases the system cost. U.S. Pat. No. 4,050,086 issued to Jerome Danforth Harr on Sept. 20, 1977 and assigned to the assignee of the present invention describes an apparatus for biasing an M-R transducer. The device alleviates the offset voltage problem. D.C. biasing current is applied to the transducer by way of a series circuitry comprising a source of unidirectional voltage and/or current, the transducer itself, and a controllable substantially constant current adjustment circuit having control input terminals.